The present invention relates to a charge coupled device image sensor and a method of making the same.
A charge coupled device (CCD) image sensor essentially comprises a light receiving region for receiving light and generating a signal charge in response to the received light, and a signal charge transfer region for transferring the generated signal charge in a single direction.
However, the light receiving region generally generates excess signals charge when too much light is incident on the light receiving region.
The excess signal charge causes a blooming phenomenon to be generated on a screen.
When a signal charge in excess of necessary signal charges is moved from the light receiving portion to the signal charge transfer region, this phenomenon is referred to as an over flow drain (OFD).
Conventionally, the light receiving region of CCD image sensor has a structure for preventing the OFD phenomenon.
The structure of the light receiving region for preventing the OFD phenomenon may generally be classified into a lateral structure and a vertical structure.
At present it is preferable to use the vertical structure, as the size of the light receiving region is reduced in accordance with the high integration of CCD image sensor.
Referring to FIG. 1, there is shown a section of a conventional CCD image sensor which has a light receiving region of the vertical structure to prevent the OFD phenomenon.
As shown in FIG. 1, the conventional CCD image sensor comprises: a silicon substrate 1 of n-type;
a well 2 of p-type formed in the surface of the silicon substrate 1; a first insulation film 3 formed on the surface of the well 2;
a signal charge transfer region 4 of n-type formed in the surface of the well 2; a surface layer 5 of p.sup.+ -type (.sup.+ denotes a high concentration) spaced from the signal charge transfer region 4 with a constant distance, for applying an initial bias;
a light receiving region 6 of n-type formed beneath the surface layer 5 of p.sup.+ -type; a first electrode 7 formed on the surface of the first insulation film 3 which is corresponding to the upper part of the signal charge transfer region 4; a second insulation film 8 formed on the surface of the first electrode 7;
a second electrode 9 formed on the surface of the second insulation film 8 and the surface of the first insulation film 3 which is located between the signal charge transfer region 4 of n-type and the light receiving region 6 of n-type;
a third insulation film 10 formed on the surface of the second electrode 9; and
a light shield layer 11 formed on the surface of the first insulation film 3 and the third insulation film 10 except for the upper portion of the light receiving region 6 of n-type.
Herein, the well 2 of p-type comprises a shallow well 2a and a deep well 2b to prevent a smear phenomenon.
The smear phenomenon means that a part of the signal charges generated from a light receiving region corresponding to a pixel are mixed with signal charges generated from another light receiving region corresponding to another pixel, and then moved to the light receiving region of n-type, corresponding to the other pixel.
Consequently, the smear phenomenon makes it impossible to correctly display an image on a screen and moreover reduces the resolution of the screen.
The operation of the CCD image sensor shown in FIG. 1 will hereinafter be described briefly in conjunction with FIG. 1.
First, when light corresponding to an image signal is incident, the light receiving region 6 generates signal charges corresponding to the image signal.
At this time, if too much light is incident on the light receiving region 6, the light receiving region 6 of n-type generates unnecessary excess signal charges in excess of the signal charges corresponding to the image signal.
At this time, if a bias is applied to the silicon substrate 1 of n-type which is connected to a voltage source Vd, the saddle point of potential taken on the line A--A' is then lowered toward the silicon substrate 1 of n-type by a predetermined height.
Therefore, the excess signal charges flow toward the silicon substrate 1, thereby enabling the OFD phenomenon and the smear phenomenon to be reduced.
However, the above-conventional technique has the following disadvantages.
Firsts the time for making a CCD image sensor is increased and the method of making a CCD image sensor is complicated, since a p-type well is essentially formed to prevent the OFD phenomenon.
Second, it is easy for the smear phenomenon to be generated and quantum efficiency is lows since the characteristic of long wavelength is not good.